1. Field of the Invention
The present invention relates to a positive temperature coefficient thermistor, and in particular, to a surface-mount positive temperature coefficient thermistor and a manufacturing method therefor.
2. Description of the Related Art
Various chip-type positive temperature coefficient thermistors capable of being surface-mounted on a printed circuit board used for the purpose of overcurrent protection have been proposed.
FIG. 21 is a sectional view showing a structural example of a conventional surface-mount positive temperature coefficient thermistor (Japanese Unexamined Patent Application Publication No. 9-232104). The surface-mount positive temperature coefficient thermistor 1 shown in FIG. 21 is formed by inserting a positive temperature coefficient thermistor element 5 having electrodes 4a and 4b provided on a respective one of the opposite main surfaces thereof, into a resin case 3a having a locking portion with a lead terminal 2a insert-molded therein, and hermitically sealing the positive temperature coefficient thermistor element 5 using a cover resin case 3b having another lead terminal 2b insert-molded therein. The electrodes 4a and 4b on the opposite main surfaces of the positive temperature coefficient thermistor element 5 and the lead terminals 2a and 2b, respectively, are brought into pressure contact with each other to establish the electrical connection therebetween.
FIG. 22 is a front view showing another structural example of a conventional surface-mount positive temperature coefficient thermistor (Japanese Unexamined Patent Application Publication No. 8-031604). The surface-mount positive temperature coefficient thermistor 11 shown in FIG. 22 is formed by inserting one terminal 13a into a space on the top-surface side within a case 12 of which three side surfaces thereof are open, inserting the other terminal 13b into a space on the grounding bottom-surface side within the case 12, and inserting a positive temperature coefficient thermistor element 14 between the pair of terminals 13a and 13b. The pair of terminals 13a and 13b, and the electrodes 15a and 15b on the positive temperature coefficient thermistor element 14, respectively, are in pressure contact to establish the electrical connection therebetween.
With the surface-mount positive temperature coefficient thermistor 1 shown in FIG. 21, (1) the manufacturing costs increase since the lead terminals 2a and 2b, respectively, are fixed to the case 3a and 3b by insert-molding; (2) the orientation of the lead terminals 2a and 2b is inconveniently determined by fixing the lead terminals 2a and 2b, respectively, to the case 3a and 3b by insert-molding; and (3) two cases 3a and 3b for the locking portion and cover portion are required in order to bring the positive temperature coefficient thermistor element 5 and the lead terminals 2a and 2b into pressure contact with each other.
On the other hand, in the surface-mount positive temperature coefficient thermistor 11 shown in FIG. 22, the terminals 13a and 13b are merely inserted into the case 12, that is, there is no need to insert-mold the terminals 13a and 13b, unlike the case of the surface-mount positive temperature coefficient thermistor 1 shown in FIG. 21. Also, it is unnecessary to use two cases in order to bring the thermistor element 14 and the terminals 13a and 13b into pressure contact.
However, with the surface-mount positive temperature coefficient thermistor 11 shown in FIG. 22, (1) since the three side surfaces thereof are open, the case 12 has reduced strength, and hence, the thickness of the case must be increased in order to maintain sufficient strength; and (2) since the position from which the terminal 13a is inserted into the case 12 is limited to one side surface, the arrangement of the terminals 13a and 13b is severely restricted, and thereby the orientation of the case when performing surface mounting is severely restricted.